Find out what size grow light for 4×4 tent setups, shelves or whole rooms you need. Enter the growing area, the wattage of each light and the plant type (30 W/sq ft for herbs to 75 W/sq ft for flowering), and the tool tells you how many lights to buy.
Grow Light Coverage Calculator
Calculate how many grow lights you need for your indoor garden area.
What Size Grow Light for a 4×4 Tent?
Multiply the 16 sq ft of a 4×4 tent by the watts per square foot your plants need. That gives about 480 W for herbs and greens, 800 W for vegging and 1,200 W for flowering. With 300 W fixtures, flowering plants need 1,200 ÷ 300 = 4 lights.
| Space | Area | Herbs (30 W/sq ft) | Veg (50 W/sq ft) | Flowering (75 W/sq ft) |
|---|---|---|---|---|
| 2×4 tent | 8 sq ft | 240 W | 400 W | 600 W |
| 4×4 tent | 16 sq ft | 480 W | 800 W | 1,200 W |
| 5×5 tent | 25 sq ft | 750 W | 1,250 W | 1,875 W |
| 10×10 room | 100 sq ft | 3,000 W | 5,000 W | 7,500 W |
- Measure the growing area: 4 × 4 = 16 sq ft.
- Pick the plant type: 75 W per sq ft for flowering.
- Find total watts: 16 × 75 = 1,200 W.
- Divide by the wattage of one light and round up: 1,200 ÷ 300 = 4.
How Many Grow Lights Do I Need for a 10×10 Room?
For vegging, 100 sq ft × 50 W = 5,000 W, so 300 W fixtures need 17 lights, which actually cover 102 sq ft. A 2×4 tent is far smaller: 600 W covers it for flowering.
How Many Watts of Grow Lights Do I Need, Really?
Use grow light wattage per square foot as a starting point, then refine. Plants respond to light, not electricity, and modern LEDs turn far more of each watt into usable light than older HID or fluorescent lamps. Count the actual power draw at the wall, not the ‘equivalent’ figure on the box, and trust a published PPF or PPFD map over watts alone.
Spread the light evenly
Total wattage can be right while coverage is still patchy. One powerful light in the middle of a large area leaves the edges dim, so several smaller fixtures often give more even results.
- Hang lights at the height the manufacturer recommends, and raise them if leaves bleach, curl upward or crisp at the tips.
- Lower the lights, or add one, if seedlings stretch and turn pale.
- Use reflective walls or a grow tent in small spaces so less light escapes.
Match the hours to the plant
Intensity and duration work together. Leafy greens and herbs are usually happy with long days under moderate light, while many flowering plants need a set dark period to trigger blooms. A timer keeps the schedule consistent and stops lights running all night by accident.
Plan for heat and power
Every watt ends up as heat in the room. In a closed tent or cupboard, a few hundred watts can push temperatures up quickly, so budget for a small fan or extractor. Check that your circuit can handle the total load, use grounded plugs, and keep cables away from water and drip trays.
Once plants are growing, let them be the judge. For background on light spectra and fixture types, see the grow light article on Wikipedia, then rerun the grow light coverage calculator whenever you change the space or the fixtures.